Positron emission particle tracking (PEPT) can be used to create images of fluid movement through geological formation core samples by tracking the location of a radioactive tag that moves through the sample with the fluid. To track and image dynamic fluid front movement, it has been experimentally determined that using a radioactive tag and an exposure of about 50 microCi is sufficient to provide a useful signal-to-noise ratio when a positron emission tomography (PET) camera is used.
This amount of exposure, which might be considered to be the lower limit of detection, corresponds to approximately 2×106 detected events per second, and assumes there is no attenuation of the radiation provided by the tag.
In a practical imaging system, the attenuation of gamma-rays between source and detector is of course not zero. Indeed, system attenuation can amount to several orders of magnitude, as gamma-rays interact with the inspected core sample, surrounding instrumentation, and the detector itself. In practical terms, this indicates a useful level of activity for the radioactive tracer tag in the core sample to be on the order of mCi.
Unfortunately, when stainless steel containers that are typically used to transport core samples are also used in imaging analysis, the number of events is attenuated even further, reducing the number of detected tag generation events to less than 2×105 per second. This amount of attenuation, which is about ten times what occurs in a non-attenuating environment, prevents useful imaging or tracking of the tagged fluid.